Stabilized zirconium oxide ceramics are used as electrolytes in oxygen sensing devices. These zirconium oxide ceramics are frequently stabilized as to one crystal structure by the addition of a stabilizing agent such as yttrium oxide, calcium oxide or ytterbium oxide. The ceramics formed by the stabilized zirconium oxide are normally covered with a porous conductive film on both sides. One side of the stabilized zirconium oxide electrolyte is exposed to a standard such as air and the other side of the ceramic is exposed to exhaust gases. The zirconium oxide is conductive for oxygen ions and by measuring changes in electric potential across the zirconium oxide ceramic it is possible to monitor the carbon monoxide content of exhaust gases. This in turn allows the electrical impulses to be fed to a computer to adjust air and fuel ratios to provide for more complete combustion. Such oxygen sensing devices have come into common use on internal combustion engines such as automobile engines to reduce polluting emissions.
The conductive film applied to the stabilized zirconium ceramic is generally platinum. The coating must be porous in order to permit oxygen ion transfer. The metallic films have been applied by flame spray techniques, sputtering techniques and the use of thick film techniques has been suggested. Flame spray and sputtering techniques are expensive. Prior thick film techniques have the disadvantage of having a low porosity. Thick film is desirable, however, from an economic standpoint as well as for uniformity of film thickness.
The present invention provides a process which allows the utilization of thick film techniques in the metallizing of stabilized zirconium oxide electrolytes while achieving high porosity of the metallized film. The present process can also be used with flame spray or sputtered metallized coatings to improve their porosity.